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Effects of Illumination Direction on the Surface Potential of CH3NH3PbI3 Perovskite Films Probed by Kelvin Probe Force Microscopy Chao Yang, Peng Du, Zhensheng Dai, Huiqin Li, Xudong Yang, and Qianli Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Applied Materials & Interfaces

Effects of Illumination Direction on the Surface Potential of CH3NH3PbI3 Perovskite Films Probed by Kelvin Probe Force Microscopy Chao Yang1, Peng Du1, Zhensheng Dai2, Huiqin Li3, Xudong Yang2,*, Qianli Chen1,2, *

1. University of Michigan – Shanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, Shanghai 200240, China 2. State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 3. Instrumental Analysis Center, Shanghai Jiao Tong University, Shanghai 200240, China

* Corresponding authors. X. Y.: Tel.: +86 21 5474 2414; Fax: +86 21 5474 2414; E-mail: [email protected] ; Q. C.: Tel: +86 21 3420 6765 ext. 5401; Fax: +86 21 3420 6525; E-mail: [email protected]

Key words: perovskite solar cell, surface potential, Kelvin probe force microscopy, illumination, photogenerated charge carriers 1

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Abstract: The development of organic-inorganic hybrid perovskite solar cells requires critical understanding in the charge carrier behaviors in the perovskite light absorbers and devices. Kelvin probe force microscopy (KPFM) has been applied as a powerful tool to probe the electrical potential distribution of perovskite films and devices, providing fundamental insights on their charge carrier properties. When measuring the material photoresponses, various approaches have been employed to illuminate the samples. Here, we measured the surface

potential

of

layer

(CH3NH3PbI3/m-TiO2/c-TiO2/FTO)

in

the and

regular inverted

mesoporous planar

structure structure

(CH3NH3PbI3/NiO/FTO) devices via KPFM. Effects of two representative illumination methods are compared - illumination from top, and from underneath through the transparent glass substrate. By comparing the variation in surface potential under two illumination methods, the surface potential of the perovskite absorbing layer in regular structure is higher than that in inverted structure. The potential difference in two structures implies that the photogenerated charge carriers are injected to the TiO2 electron transport layer and NiO hole transport layer, respectively, resulting in positive charges and negative charges accumulated in the perovskite absorbing layer. We demonstrated that the illumination direction has impact on the surface potential measurement. For the CH3NH3PbI3/TiO2 structure, illumination from underneath facilitates larger potential change. While for CH3NH3PbI3/NiO structure with insensitive photoresponse in potential change, 2


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the illumination direction has minor effect.

1. Introduction Organometal lead halide perovskites are promising materials for photovoltaic devices because of high absorption coefficient1-2, ambipolar charge tansport3-4, long carrier diffusion length4-5and tunable bandgap6. The power conversion efficiency (PCE) of hybrid perovskite solar cells (PSCs) have a tremendous increase since the first reported solid PSC of 9%7 in 2012 to a present PCE exceeding 23%8-9 within a few years, making it a competitive candidate with commercial silicon and thin film solar cells. The construction of PSC device structures are flexible, with a perovskite light absorption layer sandwiched between an electron transport material (ETM) and a hole transport material (HTM). Yet there are still a number of challenges to be solved before commercialization. To further enhance the device efficiency and long-term stability, the fundamental electrical charge carrier distribution within the devices are required to be understood. Kelvin probe force microscopy (KPFM) is a scanning probe method to image the material surface potential and electrical charge distribution with nanoscale resolution.10 By detecting the electrostatic force between the scanning probe and the sample surface, the contact potential difference (CPD) can be measured to characterize the material workfunction.11 The essential structure of KPFM is based on the atomic force microscope (AFM), which is composed of a probe supported by a 3



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cantilever beam, optical detection by a position sensitive detector on the backside of the cantilever, and a controller to provide feedback control via nullifying the error signal between the detector signal and the setpoint by applying a voltage. KPFM has been implemented as a powerful technique to characterize the surface potential distribution of perovskite films and PSC devices under different processing conditions.12-21 This method has also been applied to study the surface potential at grain boundaries of perovskite films.22-26 Li et al.25 found a band bending at the perovskite grain boundaries where electrons are accumulated. Similarly, Yun et al. observed potential barriers at grain boundaries in dark, however under illumination, the surface potential at grain boundaries are higher than potential at the grain interiors.26 These results indicate that grain boundaries are photocurrent pathways facilitating the charge transport. In addition, KPFM has provided insights on the ion migration and charge carrier behavior of perovskites under different bias voltages and various illumination conditions.22,

27-28

Joseph et al used KPFM to record real-time

CPD at nanoscale resolution under illumination and at post-illumination conditions,29 which showed that the local CPD varies in a second and recovers after several minutes due to reversible ion migration. Recent report30 demonstrated the intrinsic character of electron transport ability of SnO2 and hole transport ability of NiO via KPFM and enlarged surface potential difference between the SnO2- and NiO- based perovskite devices. The potential distribution at the cross-section of the PSC devices has

been

probed

by

KPFM

to

understand

their

4


fundamental

operating

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mechanisms.31-36 By comparing the CPD in the dark and under illumination, Cai et al. and Bergmann et al found that one diode junction exists in mesoporous PSCs, whereas the planar PSCs exhibit two diode junctions.31, 34 Illumination of samples allows the investigation of the photo-response of optoelectronic materials during KPFM measurements. Various approaches and directions have been used to apply illumination on the solar cell materials and devices. For probing the material surface, the most common approach is to illuminate from the top side of sample surface at an angle of 30º to 45º

22-23, 25-26,

to ensure the scanner

probe does not block the light pathway to the sample. The illumination has also been provided from underneath29-30,

37

through the transparent substrate. However, the

illumination from underneath is less often used, due to the limitation of the AFM instrument setup. For the measurement of device cross-section, the device is mounted vertically on a sample holder so that the tip can directly measure the device cross-section. In such measurement the device is illuminated though the transparent substrate31,

38-39

to obtain localized electronic information and workfunction

distribution across the device. Different illumination directions can result in the inhomogeneous distribution of the light-induced charge carriers, and influence the KPFM results. In most of the KPFM studies, the impact of the inhomogenous light distribution on KPFM measurements is rarely carefully investigated. Previous literatures have reported different responses of illumination on the surface potential of perovskite films based on different charge transport layers. For 5



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instance, dissimilar positive changes in CPD were discovered for perovskites on ETMs upon illumination, such as CH3NH3PbI3 (MAPbI3)/TiO2 based device (100mV)25 and MAPbI3/SnO2 based device (250mV) 30. This is surprising because the illumination intensity is lower when measuring the SnO2 based device (10 mW/cm2) than the TiO2 based device (20 mW/cm2). Importantly, we noticed that the illumination direction is different in these two works, one from the top side and the other from underneath the device, although other factors may also contribute to the measured difference, such as the ETMs and the equipment used. To ensure reliable and accurate KPFM measurement of optoelectronic materials, and in particular for the broad application of KPFM in perovskite materials and devices, appropriate illumination approach must be selected when performing the measurement. In the present work, we systematically studied the effect of illumination approaches on the CPD of the perovskite absorbing layer using KPFM. To investigate the charge separation at the interfaces of ETM and HTM, respectively, two types of device configurations

were

studied

–

the

regular

mesoporous

structure

MAPbI3/m-TiO2/c-TiO2/FTO glass (FTO), and the inverted planar structure MAPbI3/NiO/FTO. To find out the optimized illumination method, illumination was applied from top on the sample surface and underneath the glass substrate, respectively. By comparing the images obtained in the dark and under illumination, the charge distribution mechanisms of each device can be distinguished by the effects of illumination on the surface potential of perovskite films.

6


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2. Experimental Section Sample preparation. Two types of structures were prepared, namely the regular mesoporous MAPbI3/m-TiO2/c-TiO2/FTO and inverted planar MAPbI3/NiO/FTO, respectively. The sample preparation procedure follows previous reports40-41. For the MAPbI3/m-TiO2/c-TiO2 structure, a 30 nm-thick compact layer of TiO2 was deposited onto FTO substrate by spray pyrolysis. Mesoporous TiO2 layer of 100 – 150 nm thickness was spin-coated at 2000 rpm for 50s and sintered at 500 ºC for 1 hour. The MAPbI3 precursor solution was prepared by dissolving MAI (0.199g) and PbI2 (0.577) in 1ml anhydrous dimethylsulfoxide, followed by stirring at 60 ºC for 12h. 350 – 400 nm MAPbI3 perovskite film was spin-coated onto the TiO2 in glovebox by consecutive two-step spin-coating process at 1000 and 5000 rpm for 10 and 30 s, respectively, followed by annealing at 100 ºC for 10 minutes. For the inverted MAPbI3/NiO structure, acetonitrile solution of nickel acetylacetonate is spray-coated on top of FTO and annealed at 500 ºC for 30 minutes, forming NiO of 20 – 30 nm thickness. The 400 – 450 nm perovskite film was deposited on top in the glovebox using the same method above. The active area is about 10mm * 15mm. The thickness of TiO2 and perovskite layers was determined by measuring the light absorbance that is proportional to the film thickness42. The thickness of NiO layer was determined from the high resolution cross-sectional SEM image of the device43.

7



KPFM. AFM topography and CPD maps were obtained using Bruker Dimension Icon AFM in air using amplitude-modulation mode (AM-KPFM) at a lift height of 30 nm with scanning rate of 1 Hz. To prevent sample degradation in air, all measurements were performed within 0.5 h after samples were taken out from nitrogen environment. The silicon probe coated with Pt/Ir has a resonant frequency of 75kHz and force constant of 2.8N/m (PPP-EFM). CPD was recorded both in dark and under illumination. Two types of illumination sources and approaches have been used: a white LED light (Laviki, Shenzhen) from the top at an angle of 15º (Figure 1a and b), and a flat-panel LED light (HZN, Shanghai) from underneath (Figure 1c and d). In both illumination conditions, the illumination power density on the sample is maintained at 5mW/cm2 at the sample position to make sure the results are comparable.

8


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Figure 1 Schematic drawing of the KPFM setup for (a) MAPbI3/TiO2/FTO and (b) MAPbI3/NiO/FTO with illumination from the top; (c) MAPbI3/TiO2/FTO and (d) MAPbI3/NiO/FTO with illumination from underneath.

3. Results Effects of device structure The change in electrical properties of perovskite layers after exposing to light can be detected by comparison of CPD difference in dark and under illumination. Figure 1a and b illustrate the experimental setup scheme for MAPbI3/TiO2 and MAPbI3/NiO structure illuminated from the top, respectively. In Figure 2a and e, we observe the topography of MAPbI3 grains with size varying between 100 and 500 nm. When the illumination is applied from top, the topography is almost unchanged in Figure 2c and g. Mean CPD of MAPbI3/TiO2 raises from 270 mV in the dark to 9



343mV under illumination, as shown in Figure 2b and d. Whereas the CPD of MAPbI3/NiO decrease from -132mV in the dark to -175 mV under illumination in Figure 2f and h. The potential differences in Figure 2 arise because the photogenerated electrons in perovskite film are transferred to the mesoporous TiO2 ETM for MAPbI3/TiO2 structure, thus leaving positive charges in the MAPbI3 layer. For MAPbI3/NiO structure, holes are transferred to the NiO HTM and negative charges remain in the MAPbI3 layer. These results are in agreement with the previous findings23, 26, 31 which imply that photoinduced charge carriers are separated efficiently at interfaces.

10


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Figure 2 Surface potential of the two perovskite devices by KPFM with illumination from the top. (a) Topography images and (b) CPD maps of MAPbI3/TiO2 device in the dark, (c) corresponding topography and (d) CPD images under illumination. (e) Topography images and (f) CPD maps of MAPbI3/NiO device in the dark, and (g) corresponding topography and (h) CPD images under illumination.

Effects of illumination

11



For MAPbI3/TiO2, the CPD under top illumination is about 73mV higher than that in the dark. The CPD increases because more photogenerated electrons are extracted by the ETM under illumination, leaving more holes in the MAPbI3 perovskite layer, thus lift up the surface potential upon illumination. On the contrary, for MAPbI3/NiO, the CPD under illumination is 43 mV lower than that in the dark, because more holes extracted by the HTM, and more electrons trapped in the perovskite layer. As a result the surface potential in MAPbI3/NiO device is reduced under illumination. This enlarged CPD difference upon illumination between the two device structures is also known in previous reports25, 30. Furthermore, we measured the surface potential of the two aforementioned structures, this time with the illumination provided from underneath the glass substrate using a flat-panel LED light with the same power, as illustrated in Figure 1 c and d. The corresponding topography and CPD of perovskite surface are shown in Figure 3 for MAPbI3/TiO2 and MAPbI3/NiO structures, respectively. Immediately when switching on the flat-panel LED, we observe a sensitive drift in topography images induced by the heat produced by the illumination. In Figure 2 and 3, all the topography and CPD images both under top and underneath illumination were measured 500 seconds after the light source was switched on, when the lateral drift was stabilized. In this way, the temperature effect can be minimized because the prepared perovskite films are uniform and there is no difference in the mean CPD when the lateral position of cantilever is changed. The position of Figure 3c is 12


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different from Figure 3a, and this drift is originated from the thermal expansion of sample and tip upon illumination.

Figure 3 Surface potential of the two perovskite devices by KPFM with illumination from underneath. (a) Topography images and (b) CPD maps of MAPbI3/TiO2 device in the dark, (c) corresponding topography and (d) CPD images under illumination. (e) Topography images and (f) CPD maps of MAPbI3/NiO device in the dark, and (g) corresponding topography and (h) CPD images under illumination.

13



For the MAPbI3/TiO2 structure, the mean CPD increases from 270mV to 550mV; while the CPD of MAPbI3/NiO structure decreases from -60mV to -90mV upon underneath illumination. In dark condition, the CPD of NiO/perovskite has different values: -132mV (top) and -60mV (underneath) for two tests, possibly caused by the change in the probe work function during the KPFM measurement44. However, the CPD difference between dark and illumination condition for NiO/perovskite is reproducible in multiple tests using different probes and samples : the CPD difference is 38 ± 4 mV when illuminating from top; and 34 ± 11 mV from underneath.

Effects of illumination direction According to the above observation, we speculate that the density of positive charges accumulated at the MAPbI3/TiO2 interface is larger than the negative charges at MAPbI3/NiO interface. The mesoporous structure of TiO2 could exhibit better charge collection behavior than the planar NiO structure. In addition, the p-type nature of MAPbI3 could also lead to higher density of positive charges in the ETM-based MAPbI3/TiO2 than the negative charge density in the HTM-based MAPbI3/NiO, which causes large potential difference on MAPbI3/TiO2, but small potential difference on MAPbI3/NiO. To obtain an ultimate answer regarding which of the aforementioned two factors plays a more important role, we further investigated 14


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the potential difference of planar MAPbI3/TiO2 structure without mesoporous layer, shown in Figure S1 for top illumination condition, and Figure S2 for underneath illumination condition. The preparation of compact TiO2 and MAPbI3 perovskite films follows the same approach as that of the mesoporous structure. When the illumination is applied from the top, the potential increases 64 mV: from 368mV (Figure S1c) in the dark to 432 mV (Figure S1d). When the illumination is applied from underneath, the potential increases from 381 mV (Figure S2c) in the dark to 548 mV (Figure S2d). The results indicate that both factors contribute to the CPD difference between the top/underneath illumination direction. First, the different interfaces between mesoporous and planar structures: when illuminating from underneath, the CPD increase of planar MAPbI3/TiO2 (167 mV) is smaller than that of the mesoporous structure (280 mV). Second, since MAPbI3 is a known p-type material, the CPD difference in ETM-based MAPbI3 is in general larger than in the HTM-based structures. Increasing the thickness of perovskite film can enhance the light absorption, but the charge carrier transport distance also increases45. Therefore the thickness of all layers employed in this work is optimized for high efficiency and reproducible perovskite solar cells, aiming at obtaining KPFM information from optimized device structures. On the other hand, although the different optical absorption of TiO2 and NiO may also impact the KPFM result, both TiO2 and NiO only absorb UV light, whereas 90% of the white LED light is absorbed by perovskite46, therefore the impact 15



of different TiO2 and NiO thickness can be ignored. Figure 4 compares the CPD distribution with illumination from top and from underneath respectively for MAPbI3/TiO2 and MAPbI3/NiO structures. The CPD difference between MAPbI3/TiO2 and MAPbI3/NiO structures is 518mV when illuminating from top, and 639mV from underneath, smaller than the typical open-circuit voltage of 0.9-1.1V, indicating that a large portion of the photogenerated charge carriers are recombined in these semi-device, while only part of the charge carriers are transferred to the TiO2 and NiO layer.

Figure 4 The CPD distribution in the MAPbI3/TiO2 and MAPbI3/NiO devices with illumination from the top (a) and from underneath (b).

The major potential difference between two illumination methods is observed in the MAPbI3/TiO2 structure: the CPD increases 280mV when the illumination is applied from underneath, apparently larger than the CPD increase when illuminated from the top (73mV); whereas the CPD variations are similar for the MAPbI3/NiO structure. According to our observations, we propose the following mechanism that 16


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causes the potential difference in the TiO2–based structure: the accumulated positive charge density at the MAPbI3/TiO2 interface is larger than the negative charge density at MAPbI3/NiO interface. The mesostructuring would also expand the depletion region in the perovskite. Hence, there is a large electric field at the cross-section of MAPbI3/TiO2 interface facilitating the electron extraction. When illuminated from underneath, most of the photoexcited electrons are generated at the MAPbI3/TiO2 interface, as sketched in Figure 5a, followed immediately by the electron extraction into the TiO2 ETM under the built-in electrical field, as sketched in Figure 5b. On the other hand, when the illumination is provided from the top, most of the photoexcited electrons are generated on the surface of the perovskite absorbing layer in Figure 5c. These photoexcited electrons need to diffuse through at least 350-nm-thick perovskite layer before being extracted at the mesoporous TiO2 ETM. The diffusion process increases the chance of recombination within the perovskite. Therefore, less electrons are able to transfer to the TiO2 interface and slightly increase the surface potential of perovskite layer as shown in Figure 5d. On the other hand, for the MAPbI3/NiO interface with smaller accumulated charge density, the potential difference is similar at both illumination conditions.

17



Figure 5 Schematic illustration of the photogeneration (a) and charge carrier diffusion (b) process in the MAPbI3/TiO2 under illumination (a) from underneath; and schematic illustration of the photogeneration (c) and charge carrier diffusion (d) process in the MAPbI3/TiO2 under illumination from the top.

Based on our investigation, the illumination direction has a critical effect on the surface potential of optoelectronic materials probed by KPFM. This effect is prominent for the devices that are electrically sensitive to the illumination. In the previous research, researchers already demonstrated that MAPbI3/TiO2 interface is electrically sensitive to the illumination32,

34.

Herein, we suggest the following

illumination strategies for measuring surface potential of light absorbing materials such as perovskites using KPFM: in general, illumination is better to be provided from underneath, to promote the carrier separation process near the interface between perovskite and charge transport material, in particular for devices with sensitive photogenerated potential change such as MAPbI3/TiO2. Considering the thermal drift 18


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when illuminating from underneath, uniform films must be used to avoid variation in CPD due to position change. For the future works, to avoid the effect of thermal expansion on CPD, the following approaches could be tried when using underneath illumination: to place the LED from a different distance, to use a LED window with less thermal expansion coefficient or use some heat absorbing elements.

4 Conclusion In summary, we measured the surface potential in the dark and under illumination of the perovskite absorption layer in MAPbI3/TiO2 and MAPbI3/NiO structures using KPFM. The results indicate electron injection from perovskite to TiO2, and hole injection from perovskite to NiO. The potential change upon illumination in MAPbI3/TiO2 is larger than the change in MAPbI3/NiO. More importantly, by comparing the illumination directions from top and from underneath, we observe that the illumination direction can influence the CPD variations of devices with different structures. Based on our observations, we suggest to provide the illumination from underneath, especially for optoelectronic devices with sensitive photogenerated potential change such as MAPbI3/TiO2. Only when the correct illumination approach is employed can one find out the underlying mechanism from the precisely observed KPFM results.

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Funding This work was financially supported by Shanghai Sailing Program (No. 16YF1406200), the National Natural Science Foundation of China (grant no. 11574199, 11674219 and 51802193), the Programme for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

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a

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a

CH3NH3PbI3/TiO2

e

CH3NH3PbI3/NiO

b

Topography

f

Topography

c

Potential

g

Potential

dark

dark

illuminated

d

Topography

h

Topography

illuminated

Potential

Potential


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a

CH3NH3PbI3/TiO2

e

CH3NH3PbI3/NiO

Topography

f

Topography

dark

b

dark

c

Potential

g

Potential

illuminated

d

Topography

h

Topography

illuminated

Potential

Potential



a

b

Top illumination

Underneath illumination 600

600 CH3NH3PbI3/TiO2

400

illuminated

300 200 -100

73mV dark dark

500 CPD (mV)

500 CPD (mV)

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400 300 200 -100

illuminated

280mV dark dark illuminated

-200

illuminated

-200

Counts

Counts


CH3NH3PbI3/TiO2

CH3NH3PbI3/NiO

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a

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Effects of Illumination Direction on the Surface ... - ACS Publications

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